Invert slider control directions in image navigator

The control direction of the sliders and spin boxes of the image navigator
is inverted based on if the corresponding axis of the reference geometry is
flipped. The aim is that you can navigate in the same direction by dragging
the slider right, regardless whether the axis in the original image is flipped
or how it is rendered.

Dragging right means 'right', 'posterior' and 'superior'.

Signed-off-by: Miklos Espak <m.espak@ucl.ac.uk>